This invention relates to a structure for supporting an intermediate shaft for transmitting torque of a prime mover to a drive shaft of a vehicle.
Tokkai Hei 11-82653 published by the Japanese Patent Office in 1999 discloses a torque transmission device for a vehicle wherein an intermediate shaft is disposed between an output shaft of a transmission and a drive shaft of a differential, and power is transmitted between the transmission and drive shaft via an idler gear and reduction gear fixed to the intermediate shaft.
The idler gear meshes with an output gear fixed to the output shaft of the transmission, and the reduction gear meshes with a final drive gear fixed to the drive shaft. These gears are all helical gears.
Due to the meshing of these helical gears, a thrust load is mutually exerted according to the tooth trace direction. This thrust load acts by means of a gripping part of the helical gears which is offset from the rotation axis of the helical gears. Therefore, a moment acts on the intermediate shaft according to the offset distance of the application point of the thrust load. To support this moment exerted by the idler gear and reduction gear on the intermediate shaft, the intermediate shaft is supported by a tapered roller bearing which can support the thrust load and a radial load. Further, in the design stage of the torque transmission device, the moment which acts on the intermediate shaft in operation is estimated, and a prestress which opposes the estimated moment is applied to the taper roller bearing when it is fitted to the torque transmission device. This prestress cancels the moment acting on the intermediate shaft during torque transmission, and helps to reduce the noise due to the meshing of the gears.
The prestressed tapered roller bearing however increases the frictional loss of the intermediate shaft.
Further, the prestressed bearing brings another problem. The prestress is generally given by gripping a shim or spring between the outer race of the tapered roller bearing and a casing of the torque transmission device. When a shim is used, its dimensions have to be precisely controlled in order to generate a designed prestress. When a spring is used control of the prestress is easier, but an exclusive space is required to install the spring. As a result, the prestressed bearing makes the assembly of the torque transmission device complex, and leads to increase in the size of the casing.
It is therefore an object of this invention to suppress noise produced due to meshing of helical gears without applying a prestress to the bearing.
In order to achieve the above object, this invention provides a torque transmission device, comprising a first rotating shaft supporting a first helical gear, a second rotating shaft supporting a second helical gear, a rotation body comprising a third helical gear which meshes with the first helical gear and a fourth helical gear which meshes with the second helical gear, an intermediate shaft which supports the rotation body, and a thrust bearing which supports the rotation body in a direction along the intermediate shaft. The second rotating shaft and the intermediate shaft are disposed parallel to the first rotating shaft. The thrust bearing is set to have an average radius R specified by the following equation:       R    ≥                            R1          ·          R2                                      (                          R1              +                              R2                ·                H                                      )                    ⁢                                                    2                ·                H                            ⁢                              {                                                                                                    (                                                  1                          +                                                      H                            2                                                                          )                                            2                                        ·                    H                                    +                                      COS                    ⁢                                          xe2x80x83                                        ⁢                                          θ                      ⁢                      2                                                                      }                                                        ⁢              xe2x80x83            ⁢      where        ,      
    ⁢      H    =                  tan        ⁢                  xe2x80x83                ⁢        β1                    tan        ⁢                  xe2x80x83                ⁢        β2              ,
R1=pitch circle radius of the third helical gear,
R2=pitch circle radius of the fourth helical gear,
xcex21=spiral angle of the third helical gear,
xcex22=spiral angle of the fourth helical gear, and
xcex82=angle subtended by the mesh point of the first helical gear and third helical gear, and the mesh point of the second helical gear and fourth helical gear, with the intermediate shaft as center.
The details as well as other features and advantages of this invention are set forth in the remainder of the specification and are shown in the accompanying drawings.